1. Field of the Invention
The present invention relates to an amplifier circuit whose input stage comprises a gain control and whose preamplified signal output available on an active terminal of a load resistor carries a voltage the DC level of which is stabilized by associated means comprising a variable DC current generator as opposed to the average output current of the input stage and whose current is equally applied to the load resistor so that the sum of the direct currents passing through it is constant, and means which compensate for the effects of the gain control on the average operating point of the input stage.
2. Description of Related Art
The method of varying the gain of a bipolar transistor by changing the average operating point, (quiescent current) is well-known and widely used. Although it is advantageous in its simplicity, it does have the inconvenience of generating on the active terminal of the load resistor very significant DC voltage variations, which make a subsequent use of the signal very difficult, especially if one wishes to utilize a DC coupled wideband amplifier.
Thus, it has been proposed to stabilize the DC output level of the input stage by utilizing a variable DC current generator to supply a current in opposition to the average current of the amplifier transistor and also applying this current to the load resistor of the amplifier transistor so that the sum of the direct currents passing there through is constant.
An amplifier circuit of this type is known from the U.S. Pat. No. 3,678,406. It is based on the property of similarity of identical bipolar transistor characteristics and uses, more specifically, a current mirror circuit.
In so far as this similarity is not perfect, the stabilization of the DC output level of the known amplifier is not perfect and in connection herewith the subsequent signal amplification can no longer be proceeded with to a very high gain level. Furthermore, the gain variations which can be achieved are not as extensive as could be desired according to this known technique which only applies to the case in which the input stage is a bipolar transistor.
The invention has for its object to provide a variable gain amplifier circuit which does not have these inconveniences and which is not restricted to the use of a bipolar transistor for an input stage.
The invention also relates to the utilization of this circuit in an automatic gain control device.